用于混凝土、砌筑砂浆的常用水泥、用于砌筑砂浆的砌筑水泥及抹灰用水泥，它们的必试项目有什么不同?

During these ten years, many new methods have been ___ in thefield of foreign language teaching.

Which is safer-staying at home， traveling to work on public transport， or working in the office? Surprisingly， each of these carries the same risk， which is very low. However， what about flying compared to working in the chemical industry? Unfortunately， the former is 65 times riskier than the latter! In fact， the accident rate of workers in the chemical industry is less than that of almost any of human activity， and almost as safe as staying at home.

The trouble with the chemical industry is that when things go wrong they often cause death to those living nearby. It is this which makes chemical accidents so newsworthy. Fortunately， they are extremely rare. The most famous ones happened at Texas City (1947)，Flixborough (1974)， Seveso (1976)， Pemex (1984) and Bhopal (1984)。

Some of these are always in the minds of the people even though the loss of life was small. No one died at Seveso， and only 28 workers at Flixborough. The worst accident of all was Bhopal， where up to 3，000 were killed. The Texas City explosion of fertilizer killed 552. The Pemex fire at a storage plant for natural gas in the suburbs of Mexico City took 542 lives， just a month before the unfortunate event at Bhopal.

Some experts have discussed these accidents and used each accident to illustrate a particular danger. Thus the Texas City explosion was caused by tons of ammonium nitrate(硝酸铵)，which is safe unless stored in great quantity. The Flixborough fireball was the fault of management， which took risks to keep production going during essential repairs. The Seveso accident shows what happens if the local authorities lack knowledge of the danger on their doorstep. When the poisonous gas drifted over the town， local leaders were incapable of taking effective action. The Pemex fire was made worse by an overloaded site in an overcrowded suburb. The fire set off a chain reaction os exploding storage tanks. Yet， by a miracle， the two largest tanks did not explode. Had these caught fire， then 3，000 strong rescue team and fire fighters would all have died.

1.Chemical accidents are usually important enough to be reported as news because ____.

A.they are very rare

B.they often cause loss of life

C.they always occur in big cities

D.they arouse the interest of all the readers

2.Which of the following statements is true?

A.Working at the office is safer than staying at home.

B.Traverlling to work on public transport is safer than working at the office.

C.Staying at home is safer than working in the chemical industry.

D.Working in the chemical industry is safer than traveling by air.

3.According to passage， the chemical accident that caused by the fault of management happened at ____.

A.Texas city B.Flixborough C.Seveso D.Mexico City

4.From the discussion among some experts we may coclude that ____.

A.to avoid any accidents we should not repair the facilities in chemical industry

B.the local authorities should not be concerned with the production of the chemical industry

C.all these accidents could have been avoided or controlled if effective measure had been taken

D.natural gas stored in very large tanks is always safe

5.From the passage we know that ammonium nitrate is a kind of ____.

A.natural gas， which can easily catch fire

B.fertilizer， which can't be stored in a great quantity

C.poisonous substance， which can't be used in overcrowded areas

D.fuel， which is stored in large tanks

翻译：

邓小平同志曾说：“社会主义财富属于人民，社会主义的致富是全民共同致富。”构建和谐社会，就是要把民生问题作为重中之重，让广大人民群众有活干，有学上，有饭吃，有衣穿，有屋住，病有医，老有养，生活幸福，都过上好日子。自此，在就业、教育、收入分配、医疗、养老、住房等领域，一大批关注民生的实事工程接连开工，一件件事关百姓冷暖的民生大事让各族群众喜笑颜开。

某城市引水工程，输水管道为长980m、DN3500mm钢管，采用顶管法施工;工作井尺寸为 18m×20m，挖深 15m，围护结构为 φ800mm钻孔灌注桩，设四道支撑。工作井挖土前，经检测发现三根钻孔灌注桩桩身强度偏低，造成围护结构达不到设计要求。调查结果表明混凝土的粗、细骨料合格。顶管施工前，项目部把原施工组织设计确定的顶管分节长度由 6.6m改为 8.8m，仍采用原龙门吊下管方案，并准备在现场予以实施。监理工程师认为此做法违反有关规定并存在安全隐患，予以制止。顶管正常顶进过程中，随顶程增加，总顶力持续增加，在顶程达三分之一时，总顶力接近后背设计允许最大荷载。

问题

1.钻孔灌注桩桩身强度偏低的原因可能有哪些?应如何补救 ?

2.说明项目部变更施工方案的正确做法。

3.改变管节长度后，应对原龙门吊下管方案中哪些安全隐患点进行安全验算?

4.顶力随顶程持续增加的原因是什么?应采取哪些措施处理 ?